In recent years, there has been increasing interest in metal halide lamps, i.e., electric discharge devices which emit color-modified light and contain various metal halides as additives. The useful life of these lamps may range from 6,000 to over 10,000 hours, and the efficiency, measured in lumens of light per watt, of metal halide lamps in many instances is greater than 100.
In the preparation of these lamps, additive metal halides have been pulverized or powdered, and then pelleted in a desired mass or weight for machine feeding to the arc tube or envelope of the lamp. These metal halides, however, have contained impurities such as water and various hydroxides. Flaming, i.e., heating with a hand torch, has been used to drive off the water, but this procedure does not normally remove the hydroxide impurities, and has a further disadvantage in that other volatile material, desirably left in the lamp, is also driven off with the water.
The presence of hydrogen and oxygen in any form within the lamp envelope is detrimental to the lamp. These detriments may exist not only as free hydrogen and oxygen, but also so compounds thereof such as water, hydroxides, sulfates and silicates. The oxygen present within the envelope of the lamp oxidizes the metals such as tungsten which make up the lamp electrodes or filaments, and the resulting metal oxide condenses upon the interior surface of the lamp envelope thereby reducing lamp efficiency. The hydrogen present within the envelope of the lamp then reduces the oxide back to the metal freeing the oxygen to further corrode and remove more of the metal from the filament or electrode and further clouding the envelope by depositing the metal on the interior surface thereof. Thus, the lamp envelope becomes increasingly cloudy with a corresponding reduction in the efficiency of the lamp, and the metal filaments are deteriorated reducing the useful life of the lamps.
A very advantageous process and apparatus for the production of discrete particles of purified metal halides suitable for use in the production of metal halide lamps are disclosed in U.S. Pat. No. 3,676,534. As disclosed therein, a metal halide having oxygen-containing impurities is heated to a temperature above its melting point. The molten halide is purified, while at least a portion of the halide is molten, by passing therethrough at least one member selected from the group consisting of hydrogen halide, an admixture of hydrogen halide and hydrogen and an admixture of hydrogen and halogen, (the halogen, halogen radical of the hydrogen halide and the halogen radical of the metal halide being of the same halogen species) to convert at least a portion of the oxygen-containing impurities other than water present in the metal halide to water and volatile impurities and to remove at least a portion of the water and volatile impurities by scrubbing. The purified molten halide is then passed through a vibrating discharge conduit into an inert, quenching atmosphere (e.g., helium) to form particles of purified halide.
The process and apparatus of U.S. Pat. No. 3,676,534 is highly satisfactory for forming uniform pellets of metal halide salts of a relatively small size of for example, from about 200 to about 800 microns in diameter. Uniformity of size is important in accurate dosing of lamps, particularly if dosing is to be accomplished by machine operation. For the metal halides generally used in discharge lamps, particles of these sizes generally have a mass in the range of from about 0.02 to 2.0 milligrams per particle. Lamp fills usually range from about 6 to 30 milligrams per lamp.
While some lamp manufacturers prefer to use the relatively small size particles such as produced by U.S. Pat. No. 3,767,534 and secure the correct mass dosage by utilizing a measured volume of the particles, others prefer to dose with a larger pellet and its attendant lower surface area per gram. For example, if a manufacturer has lamps of three different wattages so that one lamp uses 11 milligrams of salt, another 22 milligrams of salt, and the third 33 milligrams of salt, these lamps could be dosed with 1, 2 and 3, respectively pellets of 11 milligrams mass.
However, it has been found that there are problems in forming pellets of about 3 milligrams or more in mass utilizing the process and apparatus of U.S. Pat. No. 3,676,534 since these relatively large sized particles possess too much heat of fusion to permit chilling quickly and the particles (which have a molten core) deform upon striking the bottom of the condensing chamber. Deformation of the particles into substantially flattened form decreases their utilization in commericial lamp-dosing operations. While the condenser section of the apparatus of U.S. Pat. No. 3,676,534 may be lengthened to permit further cooling of the particles and to thus alleviate the heat problem it has been found that for relatively high mass particles (e.g., 11 milligrams mass per particle), the condenser length for complete cooling of the particle is so great (e.g., equivalent to a 4 story building) as to be commercially impractical.
It has been proposed to form these relatively large-sized particles with apparatus similar to U.S. Pat. No. 3,676,534 but without a vibrating nozzle. Without vibration, the pellets drop from the discharge conduit nozzle essentially due to the hydrostatic head of the molten salt above the nozzle. The initial velocity of particles issuing from a vibrating discharge conduit is quite large while the initial velocity of pellets issuing from a non-vibrating or static discharge conduit is essentially zero. Due to the low initial velocity of pellets issuing from a static discharge conduit, the velocity of the particles through the condensation chamber is lower (thus increasing the time the particles are falling through the condensation chamber) and the final velocity of the particles at the time they reach the collection chamber is lower. Thus, particles formed from a static conduit have an increased opportunity to completely cool in the condensation chamber and a lessened tendency to become deformed upon collection.
It has further been found, however, that the molten salt may insufficiently wet the material (e.g., quartz) of the discharge conduit nozzle. If the molten salt insufficiently wets the discharge conduit nozzle, the resulting particles are quite non-uniform in size and mass. Wetting of the molten salt on the quartz discharge conduit nozzle appears to be at least partially dependent on the formation of hydrogen bonds between the oxygen of the quartz and an appropriate moiety (e.g., hydroxyl, sulfate and the like) in the molten salt. However, in the production of the ultrapure metal halide particles which have been treated to remove water, oxygen, hydrogen and any hydroxides or sulfates present, the purified molten salt is essentially devoid of the moieties necessary to effect such hydrogen bonding. It has been proposed to add a few parts per million of water to the inert quenching atmosphere which is then passed into contact with the molten salt to incorporate the water in the salt and thereby promote wetting of the molten salt. While this slight addition of water may promote wetting of the molten salt, the water also tends to react with the salt (and particularly if a lanthanide metal is a component of the molten salt) and form solid impurities which are insoluble in the salt and undesirable in the final product. Control of the amount of water necessary is also difficult and the use of too much water may lead to solid formations (e.g., scandium oxyiodide) which can clog the nozzle. The problems associated with water addition is particularly acute when the molten salt is or contains a lanthanide metal halide salt since the lanthanide metal halides form essentially insoluble salts with oxygen which interfere with the operation of the process and are detrimental impurities in the final salt product.
Accordingly, a primary object of the present invention is to provide a novel method for producing metal halides which prevent or alleviate the above discussed problems which occur in the production of metal halide lamps.
Another object of the present invention is to provide a novel method for producing relatively large sized metal halide particles containing a lathanide metal halide by passing a molten metal halide through a static discharge conduit wet by the molten halide.
Yet another object of the present invention is to provide a novel melthod for the increased wetting of molten lanthanide metal halide without substantially increasing the impurity content of the final solid lanthanide metal halide.
These and other aspects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from the claims and from the following more detailed description of a preferred embodiment when read in conjunction with the appended drawings.